Conventionally, there is a diffractive element for use in an optical head device of this kind which divides an emitted light from a laser light source into three beams including a principal beam and two sub-beams, in which the sub-beams are used for tracking control. In this usage, the diffractive element is usually one in which a material such as plastic or glass is processed into a periodic concave and convex shape, in which the depth of concave and convex is about several hundreds nm. Herein, for example, in the diffractive element which contains air in the concave portion of the concave and convex shape and glass in the convex portion, the temperature dependency of the diffraction efficiency is so small as to cause no problem in practice.
On the other hand, with a technology for integrally forming the semiconductor lasers having a plurality of oscillation wavelengths in recent years, a so-called twin LD in which the semiconductor lasers having two wavelengths of 660 nm band (for DVD) and 785 nm band (for CD) are composed as one package has been manufactured commercially. In the optical head device using the twin LD, it is expected that the number of parts in the device is reduced by sharing a part of the optical path for the laser beam of each wavelength. Supposing that the device is used in the shared optical path, a diffractive element with a wavelength selection function of the diffraction efficiency added has been proposed (e.g., refer to JP-A-2004-342295).
According to the above publication, for example, in a DVD wavelength selection diffractive element for the twin LD, the convex portion in the periodic concave and convex shape is formed of an optical multilayer for controlling the transmission phase in two wavelengths, and the concave portion is filled with a transparent material to obtain the wavelength selectivity for the diffraction efficiency. In this diffractive element, the depth of the concave and convex, or the depth of grating, is about 5 μm.
However, in the diffractive element as shown in the above publication, with a structure in which the convex portion forming the grating is formed of the optical multilayer made of inorganic material, and the concave portion is formed of an organic material, the change rate of the refractive index with the change of temperature is greatly different between the convex portion and the concave portion, whereby there is a problem that the diffraction efficiency is likely to change depending on the temperature of the system.